Each Global Positioning System (GPS) satellite in a constellation broadcasts satellite clock parameters and ephemerides and almanac information, for that satellite and for all constellation satellites, respectively, in a 50 bit-per-second stream that is received and used by a GPS signal antenna and receiver/processor, for use in determination of the time of signal receipt by, and location and velocity of, that antenna. At certain times, usually at two-hour and four-hour intervals, a GPS satellite will change its ephemeris and/or almanac parameters, its clock correction parameters and other related parameters that are included in the broadcast bit stream, in order to provide more accurate satellite information for a present time interval. During this IODE changeover transition period, which may continue for up to 90 seconds, the differential GPS (DGPS) correction information normally broadcast by a GPS reference station becomes formally unavailable, and DGPS signals from that reference station cannot be used directly for purposes of correction of the GPS signals received by a mobile GPS station from that satellite. The information content of the GPS signals and the signal changeover formalities are discussed in the ICD-GPS-200 Interface Document, published for the U.S. Government by Rockwell International Corporation, Satellite Systems Division, Revision B, 3 Jul. 1991, incorporated by reference herein.
Under the protocol adopted for Type 9 messages for the U.S. Coast Guard's Radio Technical Communication Marine (RTCM) communications of GPS information, a GPS reference station must delay transmitting information in a Type 9 message for a particular GPS satellite for a time interval of 90 seconds whenever a new Issue Of Data Ephemeris (IODE) is received by that reference station for that satellite. This time delay (90-120 seconds) is imposed to allow a target mobile GPS station, which is receiving GPS signals from that satellite and is receiving DGPS signals from that reference station, time to receive and decode the new ephemeris parameters for that satellite.
Under two early versions of the RTCM protocol, a Type 2 message was transmitted containing a range difference, computed around the time of transmission, between the ephemeris data for the new IODE regime (referred to as regime "n+1" herein) and the ephemeris data for the preceding IODE regime (referred to as regime "n" herein). Under a superseding version of the RTCM protocol, this Type 2 message is no longer transmitted. One result of this change is that a mobile station receiving RTCM messages cannot compensate for differences between the ephemeris information available and the ephemeris information needed during the changeover transition interval.
If this target mobile GPS station is using GPS and DGPS signals for that satellite (j), the DGPS information provided by the reference station during the IODE changeover transition interval is referenced to the older ephemeris data in IODE(j;n). However, the only ephemeris data available to the mobile station during the IODE changeover transition interval are the new ephemeris data in IODE(j;n+1). The target mobile GPS station can employ additional memory and double buffering, whereby ephemeride information (1) before IODE changeover and (2) after IODE changeover for that GPS satellite (j) are both stored for use during this IODE changeover transition interval. However, this requires provision of substantial extra memory within the target mobile GPS station, and this extra memory is used only during a 90-second changeover period, at most once every two hours. Space for this extra memory (a minimum of 900 bits per satellite for ephemeris and 300 bits per satellite for Universal Coordinated Time (UTC) parameters) may be unavailable in some GPS receiver/processors. Further, the associated efficiency of use of the information stored in this extra memory, a maximum of 1.25 percent, is not inspiring.
Barnard, In U.S. Pat. No. 5,119,102, discloses a vehicle location system, using GPS location determination signals and GPS satellite orbit information computed from downloaded ephemeris parameters for the satellites.
In U.S. Pat. No. 5,204,818, Landecker et al disclose survey satellite apparatus with an on-board computer that processes and compares planetary and celestial sensor data with sensor data in an on-board database, to identify any satellite misorientation or translation errors present. Satellite attitude and ephemeris are autonomously updated to reflect the present orientation and location of the satellite.
A GPS station that remembers the last-observed ephemeris data and the corresponding time of observation, when power is turned off, is disclosed by Ando et al in U.S. Pat. No. 5,222,245. If power is turned on again within a short time, the station uses the last-observed ephemeris data (before power turn-off) to estimate the present ephemeris data to process and initially estimate the present location of the station.
Mueller et al disclose a network of reference stations that track a plurality of GPS satellites and provide differential GPS corrections in U.S. Pat. No. 5,323,322. Each reference station receives GPS satellite signals, independently determines ephemeris data for each visible satellite and uses these data to provide differential GPS corrections for that satellite at that reference station.
U.S. Pat. No. 5,375,059, issued to Kyrtsos et al, discloses a GPS-assisted vehicle location determination system. Previously observed and presently observed ephemeris data for a satellite are processed to estimate the present pseudorange from that satellite to a GPS station carried on the vehicle.
A method for predicting the location of a satellite in a satellite-based navigation system is disclosed by Kyrtsos in U.S. Pat. No. 5,430,657. Orbital parameters, computed for a satellite, are used to predict a satellite location at a future time. Observed and predicted location are compared for that future time to determine whether the ephemeris data for that satellite are corrupted.
None of these approaches provides acceptable ephemeris data during an IODE changeover transition to use of new ephemeris parameters for one or more satellites. What is needed is an approach that provides RTCM Type 9 message information of acceptable accuracy during this IODE changeover transition interval, or at any other time such information may be needed. Preferably, this approach should allow use of additional computations of at most modest complexity that can be performed in parallel with the normal computations made by the target mobile GPS station to determine the station's present location, velocity and/or time of observation values ("position values"). Preferably, the amount of additional memory, if any, required for storing the additional information used in this approach should be small compared to the additional memory required for the straightforward compensation approach discussed earlier.